You are using Postfix. You are trying to send emails directly from the Postfix server. After you use telnet and helo commands, you enter a recipient like this:
rcpt to: email@example.com
But you get this error:
"550 5.1.1 firstname.lastname@example.org: Recipient address rejected: User unknown in local recipient table"
What should you do?
If you change the /etc/postfix/main.cf file to have emails use a "reply-to" of example.com, the Postfix server will think that it is the destination locally. So you will not be able to send emails to example.com provided that the "reply-to" domain uses "example.com." It is the only domain you cannot send to.
Configure the "reply-to" to be a different domain or use a different Postfix server for sending emails.
To configure a new Postfix server, see this posting if you are using Linux SUSE. See this posting if you are using a RedHat (e.g., CentOS or Fedora) distribution. For Debian/Ubuntu, we recommend trying to use the RedHat posting.